Add presence validations for unique identifiers reported from shops

[alistat]

Analytics Tracking Script will:

1. Make sure that the payload given to addOrder or addItem commands is a valid JSON before reporting them
2. Make sure addOrder has been supplied with a non-empty order_id field before reporting it
3. Make sure addItem has been supplied with non-empty order_id and product_id fields before reporting it
4. Log appropriate validation errors to console whenever the above constraints are not met

Also, Analytics Tracking Script will:

• Accept (unstringified) Object as payload. JSON.stringify(...) is not needed anymore.

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Documentation Update: skroutz/developer.skroutz.gr/pull/93

[alistat]

Closes in favor of #52